| What it is | Waterproof shell over a sleeping bag |
| Strengths | Minimal weight & bulk, adds warmth |
| Weaknesses | Cramped, condensation-prone |
| Favored by | Ultralighters, alpinists, climbers |
From bivouac, a temporary improvised camp.
A bivy sack (bivouac sack) is a waterproof-breathable shell that slips over your sleeping bag as the most minimal one-person shelter. It adds warmth and weather protection with very little weight and packed size, favored by ultralight hikers, climbers, and alpinists, but is cramped and prone to condensation.
When to use one
For fast-and-light alpine nights and ultralight trips; pair with a tarp for more weather room, over your sleeping bag.
Frequently asked questions
What is a bivy sack?
A bivy sack is a waterproof, breathable cover that goes over your sleeping bag, forming the most minimal shelter possible. It blocks wind and rain and adds a little warmth while weighing and packing far less than a tent, which is why alpinists and ultralighters use them.
When would you use a bivy instead of a tent?
When weight and packed size are critical and you accept a spartan night: fast-and-light alpine climbs, emergency or planned single bivouacs, and ultralight trips. A bivy also fits tiny ledges where no tent could pitch. For comfort, weather riding-out, or bugs, a tent is far better.
Do bivy sacks get condensation?
Yes — because you're sealed close to a single waterproof-breathable layer, moisture from your body can condense inside, dampening your bag. Venting the opening, using breathable fabrics, and pitching a small tarp over the bivy all help manage condensation.
Sources
- Alpine shelters — The Mountaineers